Boron-carbide solid state neutron detector and method of using the same

ABSTRACT

A boron carbide solid state neutron detector and method of using the detector is disclosed, wherein the detector includes a layer of boron carbide wherein the boron carbide layer is an electrically active part of the detection device, a sensing mechanism inherent to said boron carbide layer, wherein the sensing mechanism detects changes in the boron carbide layer caused by the interception of neutrons and a monitoring device coupled to the sensing mechanics.

CROSS REFERENCE TO RELATED APPLICATIONS

This application was filed under 35 U.S.C. § 371 based upon PCTApplication Number PCT/US99/28038, filed on Nov. 24, 1999 which takespriority from U.S. Provisional Application No. 60/109,898, filed on Nov.25, 1998.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The Board of Regents of the University of Nebraska acknowledges thatsome funding for the research leg to this application was provided bythe United States Government.

BACKGROUND OF THE INVENTION

The present invention relates to detection of neutrons Morespecifically, the present invention relates to a method and device forthe efficient detection of neutrons that employs a boron-richsemiconductor as an electrically active part of the detection device.

Neutron scatting is an important research method to determine thestructure of solids and liquids. It is used to understand the forcesthat act between the atoms in these systems and to determine themagnetic behavior of materials as well. The research and practicalapplications cover a broad range of areas, from the basic properties ofmaterials to studies of engineering and medical applications.

There are essentially only four elements suitable for forming solidstate semiconductor neutron detectors—boron (B), cadmium (Cd),gadolinium (Gd) and lithium (Li). Lithium semiconductor materials exist(LiInS₂, LiInSe₂ and LiZnP) but are difficult to reliably fabricate intodevices and are very difficult materials with which to work Gadoliniumconversion layer based silicon (Si) diodes have been fabricated andproposed for neutron detection, but are not particularly stable. Cadmiumzinc telluride has been shown to yield thermal neutron detection and thecadmium neutron capture cross section is high, but the neutron captureproduces such high energy gamma rays (over 0.5 MeV) that the detectorswould have to be large in order to detect these gammas efficiently.

Use of boron with neutron detectors is known both in the scintillator,the gas and the conversion layer varieties. Boron phosphide (BP)heterojunction diodes with silicon were successfully tested as alpharadiation detectors, but failed to work as neutron detectors. Boroncarbide (B₄C) was successfully used as a neutron detector based uponresistivity changes resulting from increased lithium doping, as were(111) BP wafers. The lithium production in the boron carbide was aresult of the following nuclear reactions:

¹⁰B+n→⁷Li (1.01 MeV)+⁴He (1.78 MeV)

¹⁰B+n→⁷Li (0.83 MeV)+⁴He (1.47 MeV)+γ(0.48 MeV)

Boron has also been considered as a coating to a silicon diode and aGaAs diode but the maximum efficiency is low (less than 5%).

Existing gas and liquid neutron detectors are much larger and lessrugged than solid-state ones could be. However, existing solid stateneutron detectors also suffer serious limitations. For example, knownboron doped semiconductors are only a few percent efficient because theycontain relatively little boron. Gadolinium, lithium and hydrocarbonconversion layers are all adversely affected by corrosion and hightemperatures.

Furthermore, known conversion layer devices have low efficiencies,unless multiply stacked, because the range of the reaction products inthe material of the conversion layer is generally considerably less thanthe thickness required for stopping thermal neutrons. Gadoliniumconversion layers are an exception—but the neutron—gadolinium reactionresults in conversion electrons of relatively low energy (70 keV)compared with the reaction products in the case of neutron capture byboron 10. Cadmium zinc telluride has been shown to yield thermal neutrondetection, but the neutron capture produces such high energy gamma rays(over 0.5 MeV) that the detectors must be large to detect these gammasefficiently. Scintillator combinations with photomultipliers orintensified cameras are bulky and heavy and, except forneutron-detecting scintillating fibers coupled optically to a remotephotomultiplier or camera, are intolerant of high temperatures.

Boron and boron compounds, including boron carbide, are also used inneutron absorbing shielding purposes in nuclear reactors and other typesof neutron radiation environments. For example, boron carbide can beused with shielding, thermal electric power, or detection of neutrons(by means of the resistivity change not by detection of individualneutrons). However, use of boron carbide to detect neutrons where theboron carbide is an electrically active semiconductor is novel.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an inexpensive solidstate neutron detector that includes a robust, structurally forgivingboron rich semiconductor.

It is another object of the present invention to provide a boron carbidesemiconductor that utilizes its electrical properties as a semiconductorrather than its electrical property of resistance as a means ofdetecting neutrons or its thermoelectric properties in detectingneutrons.

A still further object of the present invention is to provide adetection device that yields high gain.

A further object of the present invention is to provide a detectiondevice that provides real time response.

A further object of the present invention is to provide a detectiondevice that is capable of detecting single neutrons.

Yet another object of the present invention is to provide a detectiondevice that has low sensitivity to gamma and other radiation.

Still another object of the present invention is to provide a method ofdetecting neutrons with a detector device having a boron carbidesemiconductor.

According to the present invention, the foregoing and other objects areobtained by a detection device having a layer of boron carbide. In thedevice, the boron carbide layer is an electrically active part of thedetection device. The sensing mechanism of the detection device isinherent in the electrically connected, semiconducting boron carbidelayer, which provides neutron capture resulting in prompt, innatelyhighly amplified, electrical output signals following interception ofneutron(s).

Additional objects, advantages, and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the practice of the invention. Theobjects and advantages of the invention ray be realized and attained bymeans of the forms of instrument and the combinations particularlypointed out in the appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the accompanying drawings which form a part of the specification andwhich are to be read in conjunction therewith and in which likereference numerals are used to indicate like parts in the various views:

FIG. 1 is a schematic representation of a heterojunction diode embodyingthe present invention;

FIG. 2 is a schematic representation of the test device using theprinciples of the present invention.

FIG. 3 depicts voltage-current characteristics of heterojunction diodesof the preset invention;

FIG. 4 depicts count rates of neutrons with insertion of heterojunctiondiodes of the present invention into a neutron reactor, and

FIGS. 5 and 6 depict the relationship of ideally attainable neutrondetection efficiency as a function of the thickness of the boron-carbidelayer of heterojunction diodes of the present invention in the cases ofnatural 10 Boron abundance and 100% 10 Boron enrichment of the boroncarbide layer.

DETAILED DESCRIPTION OF THE INVENTION

Referring initially to FIG. 1, a heterojunction diode 10 is shown. Thisinvention also applies to homojunction diodes and other knownsemiconductor detection devices, examples of which are provided below.Diode 10 is shown as having a boron carbide boron-carbon alloysemiconductor 12 on a silicon substrate 14. Semiconductor 12 is grown byplasma-enhanced chemical vapor deposition (PECVD). The preferreddeposition technique is disclosed in U.S. Pat. Nos. 4,957,773 (Spenser,et al.); 5,468,978 (Dowben); 5,658,834 (Dowben), which patents areexpressly incorporated by reference herein A pair of sputter-depositedgold electrodes 16 communicate with semiconductor 12 and substrate 14.Secured to each electrode 16 is a wire 18 that serves to connectelectrodes 16 to a bias voltage source and an electrical detectiondevice such as a charge pulse measurement circuit. Thesensory/measurement devices as well as monitoring devices are known andwill not be discussed further.

Essentially, the invention works by including a boron-rich carbon alloyas an electrically active semiconductor region of a detector and byplacing the detector where it can receive neutrons. The preferred way todetect neutrons is with atoms which are the most likely to captureneutrons and in which each neutron capture leads to the creation of oneor more energetic charged particles whose mass is large compared withthat of an electron and whose energy is large and can efficiently beconverted to a measurable electrical signal. Boron atoms are highlylikely to capture neutrons and such neutron capture creates highlyenergetic ions.

The following two reactions between boron isotope 10 (¹⁰B) and a thermalneutron form the basis for neutron detection as contemplated by thepresent invention:

¹⁰B+n_(thermal)→⁷Li(0.84 MeV)+⁴He(1.47 MeV)+γ(0.48 MeV)

¹⁰B+n_(thermal)→⁷Li(1.01 MeV)+⁴He(1.78 MeV)

With a boron-rich semiconductor, the boron captures the neutron andpromptly decays into high-energy ions. The energetic ions causesecondary ionization of orders of magnitude more atoms in thesurrounding materials for each captured neutron, liberating acorrespondingly large electrical charge. The diode nature of the deviceenables the electrical charge to be collected. Also, incorporating theboron-rich alloy as an electrically active semiconductor part of thedetector allows for the overall thickness of the device to be reducedwhile retaining high efficiency of neutron detection.

The first device to use this concept was a boron-carbon alloysemiconductor (grown by plasma-enhanced chemical vapor deposition) on asilicon substrate with sputter-deposited gold electrodes, as shown inFIG. 2. As seen in FIG. 2, a boron carbide/silicon diode 20 is connectedto a charge sensitive preamplifier 22. Charge sensitive preamplifier 22,in turn, is connected to a bias voltage input 24 and a single channelanalyzer/multichannel scaler 26 which is connected to a computer 28.

In this heterojunction diode, the above reactions lead to dense localionization of atoms and hence production of electron-hole pairs (atleast of order 5×10⁵ pairs per neutron reaction), many of which arecollected due to the applied bias voltage and form a charge pulse whichis registered and counted by external circuitry. Such a device was firsttested successfully on Jul. 24, 1998 at the nuclear reactor in the VAHospital in Omaha, Nebr. This device could be improved in several ways,including ¹⁰B-enrichment (to nearly 100% ¹⁰B from the naturallyoccurring approximately 19% ¹⁰B found in unenriched boron), increasingthe thickness and quality of the boron carbide layer, changes in theelectrical configuration and electrical circuitry, and changes in thefunctional and geometrical configurations.

The deposition of films for the heterojunction diodes (boron-carbonalloy, B₅C, on (111) Si) performed in this test was undertaken in acustom designed parallel plate 13.56(MHz radio-frequency PECVD reactorused in previous studies). The silicon subrates were doped to7×10¹⁴/cm³. The (111) Si substrates surfaces were eared by Ar+ ionsputtering in the plasma reactor. The source molecule gascloso-1,2-dicarbadodecaborane (ortho-cadborane, C₂B₁₀H₁₂, was used asthe source compound for growing the boron alloy.

Typical B₅C/n-type silicon heterojunctiors have been routinely formed bythis technique. An example of one such diode device is presented in FIG.2 with the boron carbide alloy layer of about 1000 nm thick as used as aneutron detector. These devices typically have onsets of 1 eV with verylittle leakage current (less than 5 μA at 25° C.) and the boron carbidelayer has the p-type character of the undoped PECVD semiconducting boroncarbide in this device topology.

The detector area of these heterojunction diodes was about 1 cm², andwired in a “mesa” geometry. The neutron source was a small TRIGA-typereactor (V.A. Medical Center, Omaha, Nebr.) with a flux of 1.6×10⁶n/cm²·s based on calculations for the fission chamber. A heterojunctiondiode, reversed biased to about 3 V, was wired for pulse counting asshown in FIG. 2 and inserted into the reactor. The resulting count rateswith insertion are plotted in FIG. 4. Background and noise counts are inthe range of 250 to 300 Hz, and within the reactor, the count rate risesto 2×10⁵ Hz.

To assure that very little of this count rate is attributable to gammaradiation, the diode was tested against a 100 mCi ¹³⁷Cs source for gammaradiation at a distance of 10 cm. The 661 keV gamma rays provided nodetectable increase in count rate above background in spite of anexpected 10⁶ gamma rays incident on the diode per second. This isconsistent with the expected extremely low gamma-ray sensitivity of sucha solid state boron-carbon/silicon semiconductor alloy device, sinceboron and carbon have low atomic numbers and the boron-rich detectorswere made very thin (1000 nm), and the electrically active silicon layerwas under 600 nm thick.

Given that almost all counts are attributable to neutrons and that theboron carbide film is about 1000 nm thick, the detection efficiency isthus about 1% as best seen in FIG. 5. Given that devices can be madewith boron carbide of 50 micrometers to 100 micrometers in thickness andwith depletion layers extending several micrometers, the single(thermal) neutron detection efficiencies are, conservatively, expectedto reach 80% and higher in devices which simultaneously have exceedinglylow γ-ray sensitivity (<1% detection efficiency for all energies greaterthan 100 keV and <0.01% for all energies above 0.5 MeV, assured by theuse of boron as the dominant atomic species) as best seen in FIG. 6.Since the neutron—¹⁰B interaction results almost exclusively in theyield of highly ionizing lithium ions and alpha particles of totalkinetic energy about 1.5 MeV and the boron atoms form the major speciesin the active semiconducting regions of the devices, the boron-carbonalloy layer of the detector yields an enormous internal gain(considerably greater than 10⁵) which is essentially noise-free andcomparable with the gain of the intensifiers and photomultiplierscommonly used in scintillation-based detectors and imagers. By usingexclusively ¹⁰B enriched boranes in the PECVD fabrication process,detection efficiency with thinner films can be considerably improvedcompared with devices whose ¹⁰B content reflects the natural isotopicabundance, about 19% ¹⁰B.

As seen in FIG. 2, the electronics demands are minimal compared withthose for gadolinium neutron conversion layer-based detectors (whichrely on the much smaller 70 keV energetically available for signalgeneration by the conversion electrons from gadolinium), while ensuringconsiderably greater efficiency and stability. Additionally, theboron-carbon devices can be thinner than 100 μm thick and still achievenearly 100% thermal neutron detective efficiency. Stacking diodes,interleaved with neutron energy absorbers, to form efficient neutron“calorimeters” or spectrometers is also possible. In combination withboron carbide based high temperature electronics, the boron-carbon basedneutron detection systems are expected to be particularly applicable inharsh environments because of the refractory and mechanical performanceof boron carbide. The boron-carbon devices may even be fabricated onmetal substrates as well as fabricated with spatial resolution thatcould be on scales smaller than 0.5 nm. There is the possibility offabricating spatial array detectors, including position sensors forscattering experiments, as well.

High efficiency is achieved because there is a proportionally largeamount of boron present in the semiconductor layer. The boron carbidesemiconductor has boron of whatever isotope one therefore choosespresent in atomic fractions in the order of 80%. This is exceedinglyrich in boron compared with any other suitable semiconductor. Becausethe density of boron atoms in the material is so high, the boron-richlayer can be quite thin and still contain enough boron atoms per unitarea to be able to detect the neutrons very efficiently. In naturallyoccurring boron there is close to 20% of the boron atoms which are ¹⁰Batoms which are the isotopes which interact strongly with neutrons togive the reactions provided above. It is certainly possible to increasethe fraction of boron that is ¹⁰B from natural abundance to about 95% orhigher. This enrichment would result in ¹⁰B atoms accounting for afraction, about 80% or higher, of all atoms in the semiconductor boroncarbide layer. Hence, if material enriched in ¹⁰B is used rather thanjust the naturally occurring isotope ratio of ¹⁰B, the efficiencyincreases even further.

Another important issue for efficiency is not just the reaction of theneutron with boron, but the ability to detect the reaction. Byincorporating the boron atoms in an electrically active semiconductorwhere the lithium ion and the alpha particle can cause dense ionizationof other atoms, many electron-hole pairs can be created by ionization ofthe atoms, and the electric fields that can be applied across the boroncarbide layer can sweep out a large fraction of the electron-hole pairs.Thus, there are three aspects to efficiency. The first is ¹⁰B beingpresent in large number density. The second being that the reaction of¹⁰B with neutrons results in ions which very efficiently ionize atoms inthe surrounding in an electrically active semiconductor where the chargecan be swept out efficiently. The third aspect of efficiency is that ¹⁰Bresults in ions which have such a large energy that they can producevery large numbers of detectable electron hole pairs. The reactionswhich occur between neutrons and the other elements which give probableneutron interaction don't result in reaction products which are asreadily detectable or detectable to give such large signals. Boron isunique.

Another point concerns detection devices having conversion layerscontaining boron. Neutron capture by boron generates the alpha particleand the lithium ion which can only travel a very limited distance. Ifconversion layer contained enough boron atoms to cause capture of asufficient fraction of neutrons, then the layer will be so thick thatthe lithium and the alpha particles in some cases will not get out ofthe boron layer and, therefore, will not generate signals that arereadily detectable. This is a severe defect compared with the boroncarbide semiconductor devices of the present invention.

This invention can be used in various forms of solid-state neutrondetectors presenting entrance detecting areas of order μm² to m². Thesedetectors are capable of being implemented with very thin detecting andelectrically active regions (≦1 μm minimum effective electricalthickness), with very low mass per unit detecting area, withefficiencies ranging up to nearly 100% even for single neutrons, withreal-time response, with high spatial resolution (≦1 μm minimum), andwith high temporal resolution. Of course, implementation may not alwaysneed to, or be able to, employ each of these attributes. As best seen inFIG. 3, voltage and power needs are slight, as are charge pulseprocessing requirements.

Although the invention is described above as relating to heterojunctiondiodes, it is to be understood that the invention can be implemented ina large number of other ways, including homojunction diodes; p-i-ndiodes; metal-semiconductor-metal, Schottky and other diodes;transistors; diode and transistor arrays; charge-induced devices (CID)and CID arrays; charge-coupled devices (CCD) and CCD arrays; solid-stateneutron-detecting analogs of “photomultipliers”; neutron semiconductoravalanche devices; position-sensitive detectors, including those relyingon charge subdivision or sensing and on current subdivision and thosehaving capacitive or resistive means of doing so; semiconductor driftdetectors or semiconductor drift chambers; stacked series of one or moreof the above detector types which are configured to serve as neutronenergy spectrometers; individual or stacked series of one or more of theabove detector types which also, or alternatively, serve as dosimeters.The dosimeters can be capable of yielding both real-time and cumulativedosimetry information once or many times, completely nondestructively ofthe dosimetry information contained in the detectors.

The range of applicability of the present invention includes: medicalradiation dosimetry; detecting nuclear material; anti-terrorism andanti-smuggling devices; monitoring of nuclear reactors, of nuclearstorage units and facilities, and of nuclear weapons, weapons storageand weapons shipment; life science materials and physical sciencesscattering experiments; monitoring of neutron sources; calibration ofneutron flux; personnel and environmental radiation protection;radiation protection at high energy radiation facilities, includingmedical x-ray facilities (high energy ones); neutron cancer therapy;profiling of medical, therapeutic, research and other neutron beams;comet, planetary and other space exploration.

From the foregoing, it will be seen that this invention is one welladapted to attain all the ends and objects herein above set forthtogether with other advantages which are obvious and which are inherentto the structure. It will be understood that certain features andsubcombinations are of utility and may be employed without reference toother features and subcombinations. This is contemplated by and iswithin the scope of the claims.

Since many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

What is claimed is:
 1. A neutron detection device, said devicecomprising: a sensing mechanism, said sensing mechanism having a layerof boron carbide semiconductor wherein the boron carbide layer is anelectrically active part of said detection device; and a monitoringdevice, wherein said monitoring device records changes in said boroncarbide layer detected by said sensing mechanism.
 2. The device of claim1, wherein said sensing mechanism is inherent in said boron carbidesemiconductor layer and results in a prompt, innately highly amplified,electrical output following capture of a single neutron.
 3. The deviceof claim 2, wherein said device is a homojunction diode.
 4. The deviceof claim 1, further comprising a layer of silicon communicating withsaid layer of boron carbide.
 5. The device of claim 4, wherein saiddevice is a heterojunction diode.
 6. The device of claim 1, wherein thethickness of said boron carbide layer is about 1000 nm.
 7. The device ofclaim 5, wherein the thickness of said silicon layer is less than 600nm.
 8. The device of claim 1, further comprising at least two diodesinterleaved with a neutron energy absorber.
 9. The device of claim 1,wherein said boron carbide layer is fabricated on a metal substrate. 10.The device of claim 1, wherein said boron carbide layer contains atleast 80% ¹⁰B.
 11. The device of claim 1, wherein said device is capableof operating at 500° C.
 12. A method of detecting neutrons, said methodcomprising: positioning a neutron detecting device in a location toallow said device to intercept a stream of neutrons, said neutrondetecting device comprising a layer of boron carbide wherein said boroncarbide layer is an electrically active part of said device, and asensing mechanism coupled to said boron carbide layer; introducing atleast one neutron traveling in a direction to be intercepted by theboron carbide layer; and monitoring the interaction of the neutron withthe boron carbide semiconductor; wherein said sensing mechanism detectschanges in said boron carbide layer caused by the interception ofneutrons.
 13. A method of detecting neutrons, said method comprising:positioning a neutron detecting device in a location to allow saiddevice to intercept a stream of neutrons, said neutron detecting devicecomprising a layer of boron carbide wherein said boron carbide layer isan electrically active part of said device, and a sensing mechanisminherent to said boron carbide layer; introducing at least one neutrontraveling in a direction to be intercepted by the boron carbide layer;and monitoring the interaction of the neutron with the boron carbidesemiconductor; wherein said sensing mechanism detects changes in saidboron carbide layer caused by the interception of neutrons.
 14. Aneutron detecting device comprising: a semiconducting boron carbidelayer; and a substrate layer coupled with the semiconducting boroncarbide layer, wherein the semiconducting boron carbide layer is anelectrically active region of the detecting device.
 15. The neutrondetecting device of claim 14, further comprising: at least twoelectrodes, wherein one electrode is coupled with the semiconductingboron carbide layer, and wherein the other electrode is coupled with thesubstrate layer.
 16. The neutron detecting device of claim 15, furthercomprising: a bias voltage source; and an electrical detection device,wherein the bias voltage source and the electrical detection device arecoupled with the two electrodes.
 17. The neutron detecting device ofclaim 14, wherein the substrate is formed of silicon.
 18. The neutrondetecting device of claim 14, wherein the substrate is formed of metal.19. The neutron detecting device of claim 14, wherein the semiconductingboron carbide layer is p-type.
 20. The neutron detecting device of claim19, wherein the substrate layer is n-type.
 21. The neutron detectingdevice of claim 14, wherein the semiconducting boron carbide layercontains at least 80% ¹⁰B.